The present invention relates to a method for the treatment of a catalytic component of the Ziegler type which is deposited on an inorganic support of the porous metal oxide type which preserves the catalyst's morphology and control over the morphology of the polymer resulting from gas-phase polymerization of ethylene or gas-phase copolymerization of ethylene and at least one alpha olefin containing 3 to 12 carbon atoms.
It is known to deposit catalytic components on a support based on a porous metal oxide and to prepolymerize ethylene in the presence of said catalytic components. These two methods are used to regulate the size of the system injected in a gas-phase polymerization reactor. However, their combination alone does not permit control of particle size, elimination of the formation of fine particles consisting of particles which broke off the growing polymer, or an improvement in the comonomer's effectiveness in reducing the density of the crystallinity ratio (comonomer efficiency).
There also exist high-pressure polymerization processes which are different from gas-phase polymerization processes. They consist of depositing a catalytic component on a porous metal support and in prepolymerizing an alpha olefin on this combination. This is the case for the technique described in EP 133,383, according to which, after deposition on a porous metal support, the catalytic component is prepolymerized in contact with a C.sub.4 to C.sub.18 alpha olefin in the presence of a noncomplexed alkylaluminum. Under these conditions, a catalyst is produced with an average granulometry of less than 7 microns. This is difficult to isolate since the size is too fine. It is completely unsuited for gas-phase polymerization in which, by contrast, a catalyst must be used with as few fine particles as possible.
According to French Patent 2,566,782, a catalytic component without support based on a porous metal oxide is used directly in fluidized polymerization and entails risks of point heat peaks which can lead to the formation of agglomerates, to the setting of masses, and, in some instances, to the breaking up of the catalyst with loss of morphology and formation of fine particles which are always dangerous in a fluidized bed.
To correct these disadvantages, the catalytic component can be combined with a resistant granular support such as alumina, silica, manganese, or aluminum silicate. However, as specified in French Patent 2,566,782, the granular support increases the activity of the catalytic system with the consequences mentioned above. To avoid these consequences, the amount of support is increased, leading to a relatively high amount of inorganic residue in the final polymer.
The catalytic component without a support based on porous metal oxide can also be converted into a prepolymer. But, as in the preceding case, a decrease in the activity of the component requires an increase in the amount of prepolymer. According to French Patent 2,566,782, this reduces the economic advantage of the gas-phase polymerization process by complicating the prepolymerization phase due to the increase in the amount of solvent to be retreated and the need for large storage installations.
To avoid these disadvantages, the recommendation is made in French Patent 2,566,782 to use hydrogen for the treatment of the catalytic component, which is in the form of the prepolymer or a deposit on a support based on a porous metal oxide in the presence of a cocatalyst before using the catalytic component for polymerization in fluidized bed, also called gas-phase polymerization. This technique presents the disadvantage of deactivating the catalytic systems or the prepolymers and consequently results in a loss in productivity.
This technique, which consists of prepolymerizing ethylene in the presence of a catalytic component deposited on a porous metal oxide support in the presence of only alkylaluminum, is used again in EP 174,104. This document confirms that during prepolymerization particles break off, and under the described conditions it is impossible to preserve the morphology of the catalytic component in the prepolymer, and thus in the final polymer, when the active prepolymer is used for the polymerization of ethylene. According to the technique of EP 174,104, it is explicitly stated that the prepolymerized component is a powder with a particle size smaller than that of the nonprepolymerized component.